Systems for ensuring safety and reliability in aircraft maintenance organisations do not work as they are supposed to do. This does not necessarily mean that these organisations are not safe, because unofficial and informal mechanisms compensate for these organisational deficiencies. Organisations find it difficult to manage these unofficial patterns of normal behaviour when they occur in incidents. This leads to 'cycles of stability', in which important lessons for prevention are not learned. Similar underlying patterns of culture and action may explain three things: why such systems are highly resilient, reliable and comparatively safe despite their systemic deficiencies; why they are also vulnerable to occasional failure; and why they are highly resistant to change, even following failure.